Microcirculatory permeability of the dog heart on cardiopulmonary bypass has been studied with the heart beating and fibrillating. Indicator dilution techniques were used to measure the extraction (E) and permeability surface area product (PS) of 14C-inulin and 125I-albumin using 99mTc red cells as the reference tracer. Fibrillation resulted in increased E and PS of inulin. Coronary flow increased while vascular resistance and oxygen extraction fell. A similar model was used to study the effects of ischemia and reperfusion. This resulted in increased PS of inulin and increased permeability to albumin at 45 minutes of reperfusion. Studies are continuing on the effect of cold blood cardioplegia on microvascular permeability using similar techniques. In addition, the isolated rat heart is being used to investigate the transport of different sized ultrastructural probes across the capillary endothelium.